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NEET CHEMISTRYMedium

Reversible expansion of an ideal gas under isothermal and adiabatic conditions are shown in the figure: ABIsothermal expansion\text{AB} \rightarrow \text{Isothermal expansion}, ACAdiabatic expansion\text{AC} \rightarrow \text{Adiabatic expansion}. Which of the following options is not correct?

A

ΔSisothermal>ΔSadiabatic\Delta S_{\text{isothermal}} > \Delta S_{\text{adiabatic}}

B

TA=TBT_A = T_B

C

Wisothermal>WadiabaticW_{\text{isothermal}} > W_{\text{adiabatic}}

D

TC>TAT_C > T_A

Step-by-Step Solution

Let us analyze each option for the expansion of an ideal gas from the same initial state A:

  1. Isothermal expansion (A \rightarrow B): By definition, the temperature remains constant throughout the process. Therefore, TA=TBT_A = T_B.
  2. Adiabatic expansion (A \rightarrow C): In an adiabatic expansion, the system does work at the expense of its internal energy (q=0q = 0, so ΔU=W\Delta U = W). Since the internal energy decreases, the temperature of the ideal gas must drop. This means TC<TAT_C < T_A. Thus, the statement TC>TAT_C > T_A is incorrect.
  3. Entropy change (ΔS\Delta S): For a reversible adiabatic process, heat exchange is zero (qrev=0q_{rev} = 0), so the entropy change ΔSadiabatic=0\Delta S_{\text{adiabatic}} = 0. For an isothermal expansion, the volume increases, leading to an increase in randomness, so ΔSisothermal>0\Delta S_{\text{isothermal}} > 0. Therefore, ΔSisothermal>ΔSadiabatic\Delta S_{\text{isothermal}} > \Delta S_{\text{adiabatic}}.
  4. Work done (WW): In a P-V diagram, the isothermal curve lies above the adiabatic curve for expansion from the same state. Thus, the area under the isothermal curve is greater, meaning the magnitude of work done by the gas is larger: Wisothermal>Wadiabatic|W_{\text{isothermal}}| > |W_{\text{adiabatic}}|.
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